Group III nitride semiconductors are utilized as functional materials for constructing Group III nitride semiconductor light emitting devices having a pn junction type configuration such as light emitting diodes (LEDs) or laser diodes (LD), which emit short-wavelength visible light (for example, see Japanese Unexamined Patent Publication No. 2000-332364). For example, when constructing an LED which displays emitted light in the near ultraviolet band, the blue band or the green band, n-type or p-type aluminum gallium nitride (compositional formula AlxGayN: 0≦x, y≦1, x+y=1) is utilized to construct the clad layer (for example, see Japanese Unexamined Patent Publication No. 2003-229645). Also, gallium indium nitride (compositional formula: GayInzN: 0≦y, z≦1, y+z=1) is utilized for producing active layers (light emitting layers) (for example, Japanese Examined Patent Publication SHO No. 55-3834).
For conventional Group III nitride semiconductor light emitting devices, it is common to bond an n-type or p-type Group III nitride semiconductor layer to the light emitting layer. This creates a light emitting section with a hetero junction structure for high-intensity light emission. For example, in order to construct a light emitting section with a double hetero (DH) junction structure, the light emitting layer is conventionally composed of GayInzN (0≦y, z≦1, y+z=1), and an n-type or p-type Group III nitride semiconductor layer is bonded as the clad layer (for example, see I. Akazaki, “Group III-V Compound Semiconductors”, May 20, 1995, Baifukan Publishing, Ch. 13.).
An n-type Group III nitride semiconductor layer situated between a substrate and a light emitting layer, for example, is conventionally constructed entirely of a Group III nitride semiconductor containing added silicon (Si). For example, an n-type AlxGayN (0≦x, y≦1, x+y=1) layer is utilized which has its resistivity controlled by adjustment of the silicon doping content (for example, see Japanese Patent No. 3383242).
However, when silicon (Si) is doped in a large amount in an attempt to accomplish vapor phase growth of a low-resistance n-type Group III nitride semiconductor layer, cracking has been a common problem (for example, see H. Murakami et al., J. Crystal Growth, 115(1991), 648). Specifically, in the conventional techniques for silicon doping, it has not been possible to stably obtain an n-type Group III nitride semiconductor layer with low resistance and in a continuous manner.
On the other hand, Ge is known as an n-type impurity alternative to silicon (Si) (for example, see Japanese Unexamined Patent Publication No. 4-170397). However, it has low doping efficiency compared to Si (see Jpn. J. Appl. Phys., 31(9A) (1992), 2883), and is unsuitable for obtaining low resistance n-type Group III nitride semiconductor layers. In addition, high-concentration doping of Ge has been problematic due to generation of pits in the surface of the n-type Group III nitride semiconductor layer, which impair its flatness (see Group III Nitride Semiconductor Compounds (CLARENDON Press (OXFORD) 1998), p. 104.).